Alkenoic acid derivative products obtainable by the present process, such as alkenoic esters, acids and anhydrides, constitute versatile intermediates for various chemical products. They become particularly accessible if cheap base feedstock, such as butadiene can be used. Alkyl 3-pentenoates, for example, are intermediates to dialkyl adipates and 6-oxohexanoates in an efficient manufacturing route to polyamide-6,6 and polyamide-6, respectively.
U.S. Pat. No. 4,172,087 discloses a process for the carbonylation of conjugated dienes in the presence of an alcohol and using a catalyst system containing a halide-free palladium salt, a multidentate phosphine ligand and tertiary nitrogen base (e.g., pyridine, quinoline or isoquinoline). The tertiary nitrogen base is used in large excess relative to the other catalyst system components, and essentially serves as the solvent for the reaction. In the absence of added N-heterocyclic solvent, considerable precipitation of palladium-containing insoluble species occurs and much lower product yields are reported. This known process involves two concurrent reactions, viz. dimerization and carbonylation, and mixed reaction products (e.g., isopropyl pentenoate and isopropyl nonadienoate) were obtained.
Subsequent improvement in terms of selectivity of the conjugated diene carbonylation was disclosed by EP-A-273489. In EP-273489, the reaction is carried out in the presence of a specific substantially organic nitrogen-containing base-free catalyst system containing a palladium compound in conjunction with at least one multidentate organic phosphorus ligand. Specifically in a preferred embodiment, a catalytic quantity of a protonic acid with a pKa value&gt;3 is added to increase the yield of, for example, pentenoates in the case of conversion of butadiene emphasizing the desirability of avoiding the basic reaction medium used according to U.S. Pat. No. 4,172,087. The process of EP-A-273489 affords the carbonylation of butadiene to proceed with a selectivity to alkyl pentenoates of about 90% or higher.
The known processes provide an interesting route to alkenoic acid derivatives from cheap feedstock, but their reaction rates in relation to the amount of precious palladium catalyst component applied leaves room for further improvement for providing an industrially viable process. In particular, for the production of polyamides further improvement is necessary to compete with commercial technology.
It is therefore an object of the present invention to provide an improved process to produce alkenoic acid derivatives from aliphatic conjugated dienes.